1. Field of the Invention
This invention relates generally to systems for coating web materials such as an apparatus for lacquering print materials in a production line of a printing plant; in particular, the invention deals with a closed loop recirculating coating liquid supply system with viscosity adjustment.
2. Related History
A typical example of a lacquering assembly suitable for integration in a production line of a printing plant was disclosed in French patent document 2,590,842. The lacquering assembly illustrated therein included a scoop roll which drew a lacquer coating mixture from an application trough and transferred the lacquer coating mixture to application rolls for continuous coating of a running web of print material.
In general the application of a lacquer coating or coatings to print material resulted in a high quality print job with optical properties otherwise unattainable in single color printing or successive overprinting. The lacquer coating imparted abrasion resistance, retarded blocking when imprinted materials were stacked and facilitated immediate further processing of imprinted materials. In order to assure uniform high quality coated print materials, lacquering assemblies were required to be furnished with a supply of lacquer coating mixture of specified rheological values which were to be constant throughout a particular job or run. Variations in properties such as viscosity were required to be quickly adjusted to meet the specifications of the job or run.
Because the lacquer coating mixture supplied to the rolls of the lacquering assembly comprised several constituents, each of which were characterized with different vaporization properties, it was necessary to monitor flow properties of the lacquer coating mixture to provide adjustment in the event such flow properties deviated from specified viscosity limits. Changes in viscosity were a function of several parameters, including surface evaporation, temperature changes and impurities introduced into the system.
Deviations from specified viscosity limits of a lacquer coating mixture resulted in impairment of the quality of the lacquer film or coating applied to the print material. Deviations in viscosity also changed the absorption characteristics of the lacquer coating mixture relative to its interaction with the various rollers of the lacquering assembly and thus impaired the operating characteristics of the lacquering assembly itself.
As previously mentioned, the lacquering assembly illustrated in French patent document 2,590,842 included an applicator trough carrying a quantity of lacquer coating mixture and a scoop roller rotating in the applicator trough. The scoop roller drew the lacquer coating mixture from the trough and coated it on the print material either directly or through an applicator roll. The lacquering assembly further operated in conjunction with a lacquer supply system which fed a lacquer mixture from a thermostatically controlled tank to the applicator trough.
The thermostatically controlled tank included a viscometer coupled to a valve which was controllable to permit a suitable quantity of thinner to be added into the tank and mixed with the lacquer mixture in the tank for the purpose of adjusting the viscosity of the coating lacquer mixture. Lacquer mixture, adjusted as to viscosity, was pumped from the tank to the applicator trough. A return line was provided from the applicator trough to the tank.
The viscometer comprised a discrete measuring device positioned in the tank from above and operated on a measuring principal common with laboratory viscometers including falling ball viscometers and rotation viscometers. Among the disadvantages inherent in such system was the fact that the viscosity measurement represented a sampling from only a specific point in the tank and the possibility of overadjustment of underadjustment of the viscosity of the lacquer coating mixture in the applicator trough was present since the viscosity of the lacquer mixture at a particular point in the tank was not necessarily that of the lacquer mixture in any other portion of the tank, in the feed lines or in the applicator trough.
In U.S. Pat. No. 4,552,165 issued to LUSKA, a system was disclosed for supplying ink and for maintaining the density of a printed color constant by detecting reflected light rays from a printed mark to provide a correction signal which was combined with a viscosity code signal obtained from a viscometer positioned at a specific location within a tank carrying an ink/solvent mixture. Such system suffered disadvantage of potential improper viscosity adjustments for the same reason as the system disclosed in French patent 2,590,842.